1. Field of the Invention
This invention relates to improvements of the Gauss type objective widely used as the standard lens for single lens reflex cameras, and, more particularly, to a Gauss type objective employing a quick and easy focusing method.
2. Description of the Prior Art
In the past, the so-called Gauss type lens has found wide use as the standard lens for single lens reflex cameras.
This was because, at a total angular field coverage on the order of 45.degree., good image quality could be relatively easily assured over the entire area of the picture format, and also because it was relatively easy to make the lens faster than 2 in F-number. A further reason was because it was also easy to secure a relatively long back focal distance as is required for the single lens reflex camera.
Most of the Gauss type objectives employ the focusing method of moving the entire lens system in unison with the diaphragm.
An alternative method of focusing by moving part of the lens system has been proposed as, for example, in U.S. Pat. No. 250,389. But it is difficult to say that it gives a sufficient performance to the objective for the single lens reflex camera. As far as the Gauss type objectives are concerned, to obtain a good imaging performance and a sufficient back focal distance, the following conditions are generally satisfied: EQU 2f&lt;f.sub.F &lt;3f, 0.8f&lt;f.sub.R &lt;1.2f
where f is the focal length of the entire lens system; and f.sub.F and f.sub.R are focal lengths of the front and rear lens groups with the diaphragm therebetween. An example of the objective having such power distribution is known in Japanese Laid Open Patent Sho No. 52-146620. In the practical embodiment of this patent, those of the lens members which lie in rear of the diaphragm are made movable for focusing down to as short an object distance as 16f without causing mechanical interference with the front lens group ahead the diaphragm.
FIGS. 1a to 1e and FIGS. 2a to 2e are graphic representations of the various aberrations of the typical Gauss type objective when focused to an infinitely distant object and an object at a distance of 16f respectively. It is evident from these aberration curves that as focusing is effected down to shorter object distances, the spherical aberration becomes under-corrected, and the outward coma is objectionably increased in the intermediate to marginal zone of the picture format. Even when the diaphragm is closed down this outward coma still remains appreciable, giving its influence on the imaging performance. On the other hand, recently in the art of photographic cameras, an automatic focusing technique has been developed and is in progress, and even some of the single lens reflex cameras come to employ automatic focusing control mechanisms. In this case, if the focusing method is by bodily moving the objective lens, a large stress is laid on the lens operating mechanism. To avoid such disadvantage, the mass of the movable lens is required to be as light as possible. In this point of view, the method of focusing by moving part of the lens system is desired.
As the Gauss type objective having the focusing provision in part of the rear lens group thereof, mention may be made of the ones disclosed in Japanese Laid Open Patent Sho Nos. 56-27108 and 56-114918.